The present invention relates to seat belt retractors used in seat belt apparatuses which are installed in vehicles such as automobiles, etc., for restraining and protecting an occupant by a webbing. The seat belt retractors allow a free extraction and retraction of the webbing in a normal situation, and stop the extraction of the webbing in an emergency, for example, when a large deceleration occurs from a vehicle collision. More specifically, the present invention relates to a seat belt retractor including an energy absorbing mechanism (which will also be referred to as an EA (xe2x80x9cEnergy Absorbingxe2x80x9d) mechanism in the following descriptions), which, when the extraction of the webbing is stopped in an emergency, absorbs an impact energy and limits a load applied to the webbing within a predetermined load limit.
Conventionally, seat belt apparatuses are installed in vehicles such as automobiles, etc., for restraining and protecting an occupant by a webbing. A typical seat belt apparatus includes a seat belt retractor which allows free extraction and retraction of the webbing in a normal situation, and stops the extraction of the webbing in an emergency as described above.
Various seat belt retractors have been suggested which include an EA mechanism, which, when the extraction of the webbing is stopped in an emergency, absorbs an impact energy caused by an inertial movement of an occupant, and limits a load applied to the webbing within a predetermined load limit. As an example of the EA mechanism which limits the load applied to the webbing, there is a mechanism in which a torsion bar is attached to a spool provided for winding the webbing in such a manner that the torsion bar and the spool have the same rotational center. When the extraction of the webbing is stopped in an emergency, the torsion bar is twisted so that the energy is absorbed and the load applied to the webbing is limited.
In the EA mechanism of seat belt retractors, an EA load is preferably controlled in accordance with various conditions including a physical size of an occupant, velocity of a vehicle at the moment of collision, etc., for absorbing an impact energy applied to an occupant more effectively.
However, in the above-described conventional seat belt retractors, the load limited by the EA mechanism (the EA load) is set to a constant value. Accordingly, it is difficult for the conventional seat belt retractors to reliably satisfy the above-described requirement to absorb the energy flexibly.
Accordingly, in view of the above-described situation, an object of the present invention is to provide a seat belt retractor including an EA mechanism which can reliably satisfy the above-described requirement to absorb the energy flexibly.
According to the present invention, a seat belt retractor which, in a normal situation, allows a free extraction and retraction of a webbing which is wound around a spool, and which, in an emergency, restrains an occupant by stopping the extraction of the webbing is provided. The retractor includes an energy absorbing mechanism which, in the emergency, limits a load applied to the webbing within a load limit. The mechanism has a load limiting characteristic such that the load on the webbing is gradually reduced after reaching the load limit.
In addition, according to another aspect of the present invention, the energy absorbing mechanism includes a torsion bar which is twisted by a torque caused by an extraction force applied to the webbing in the emergency. The bar absorbs an impact energy which occurs in the emergency. The torque is gradually reduced so that, in the load limiting characteristic of the energy absorbing mechanism, the load on the webbing is gradually reduced after reaching the load limit.
In addition, according to still another aspect of the present invention, a circular pulley is attached to the torsion bar in a decentered (i.e. off-centered) manner and a wire is wound around the circular pulley. The wire is pulled by a force caused by the extraction force applied to the webbing, so that the torque is gradually reduced.
In addition, according to yet another aspect of the present invention, the energy absorbing mechanism includes a wire of which the diameter is gradually reduced and paired members which sandwich and compress the wire. The wire is pulled by a force caused by the extraction force applied to the webbing in the emergency. The compression force applied to the wire by the paired members is gradually reduced, so that, in the load limiting characteristic of the energy absorbing mechanism, the load on the webbing is gradually reduced after reaching the load limit.
According to another aspect of the present invention, the energy absorbing mechanism includes a wire having a constant diameter and paired members which sandwich and compress the wire and which are arranged in such a manner that a gap therebetween varies. The wire is pulled, while the gap between the paired members is increased, by a force caused by the extraction force applied to the webbing in the emergency. The compression force applied to the wire by the paired members is gradually reduced, so that, in the load limiting characteristic of the energy absorbing mechanism, the load on the webbing is gradually reduced after reaching the load limit.
The energy absorbing mechanism may comprise a wire having constant diameter and a load limiting member which has a meandered guide groove in which the wire is fitted. The wire is pulled by a force caused by the extraction force applied to the webbing in the emergency and a length of a part of the wire which remains inside the meandered guide groove is reduced. As a result, in the load limiting characteristic of the energy absorbing mechanism, the load on the webbing is gradually reduced after reaching the load limit.
According to another aspect of the present invention, the energy absorbing mechanism includes a torsion bar which is twisted by a torque caused by an extraction force applied to the webbing in the emergency, and which absorbs an impact energy which occurs in the emergency. The mechanism further includes a pulley which is attached to the torsion bar and which has such an exterior that a rotational radius, which is the distance between the rotational center and the periphery, varies. The retractor also includes a wire which is wound around the pulley and is connected to the pulley at an end thereof. The wire is pulled by a force caused by the extraction force applied to the webbing and the pulley is rotated so that the torque is applied to the torsion bar. In addition, the rotational radius of the pulley varies so as to gradually reduce the torque, so that, in the load limiting characteristic of the energy absorbing mechanism, the load on the webbing is gradually reduced after reaching the load limit.
In addition, according to yet another aspect of the present invention, the energy absorbing mechanism includes a torsion bar which is attached to the spool in such a manner that the torsion bar and the spool have the same rotational center. The energy absorbing mechanism has a load limiting characteristic which is the sum of a load limiting characteristic obtained from the torsion bar attached to the spool and the load limiting characteristic in which the load is gradually reduced after reaching the load limit.
In a seat belt apparatus in which the present invention is applied, the energy absorbing mechanism has a load limiting characteristic such that the load on the webbing is gradually reduced after reaching the load limit. Thus, the load applied to an occupant by the webbing W during a vehicle collision is gradually reduced with time. Accordingly, by setting the inclination of the load reduction in the load limiting characteristics, the requirement to absorb the energy flexibly may be reliably satisfied.
The energy absorbing mechanism is constructed with a torsion bar, a pulley, and a wire. In addition, according to other aspects of the present invention, the energy absorbing mechanism may be constructed with a wire and paired members which sandwich and compress the wire. Alternatively, the energy absorbing mechanism may be constructed with a wire and a load limiting member which has a meandered guide groove in which the wire is fitted. In either case, the energy absorbing mechanism having a simple construction may be realized at a low cost.
The energy absorbing mechanism may have a total load limiting characteristic which is the sum of the two load limiting characteristics. Accordingly, load limiting characteristic may be flexibly set, so that the requirement to flexibly absorb the energy may be reliably satisfied.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.